A Theoretical Study On Doubly Ionic Hydrogen Bonds Based On Imidazolium Cations

1 Ionic liquids with unique properties has attracted people's attention,and it has been widely applied.Hydrogen bonds is an important non-covalent bond.It's number and strength will influence the properties of ionic liquids.In the paper,the structures of ion-pairs and Hydrogen bonds between 1-methylimidazolium cation [Mim]+?1,3-Dimethyl-imidazolium cation [Mmim]+ and inorganic anions NO3-,ClO4-,BF4-,AlCl4-and Al2Cl7-were systematically studied using quantum chemical calculation methods.(1)Structure optimization and properties calculation for 1-methylimidazolium cation [Mim]+ complexes,including [Mim]+[NO3]-? [Mim]+[ClO4]-? [Mim]+[BF4]-? [Mim]+[AlCl4]-?[Mim]+[Al2Cl7]-,have been performed using the MP2 method with the 6-311++G(d,p)basis set.The geometric and energetic features,the second order perturbation analysis in Natural bond orbital(NBO),charge transfer from anions to the cation and atoms in molecules(AIM)theory have been carried out and discussed for these complexes.The results show that: a.charge transfer from anion to the cation take s place in all complexes and that verifies hydrogen bonds exist between anions and the [Mim]+ cation;b.comparing the hydrogen bond lengths,the second order perturbation energy E(2)and electron densities ? of bond critical points(BCP),it is proved that the protic NH-X(X=O,F,Cl)H-bonds is relative larger than the aprotic CH-X(X=O,F,Cl)H-bonds;c.the total interactions are correlated to the strength of the hydrogen bonds for these complexes,and the order of the total interactions and the hydrogen bonds is: [Mim]+[NO3]-> [Mim]+[ClO4]-> [Mim]+[BF4]-> [Mim]+[AlCl4]-.d.in all the complexes,the “side” conformers,in which anions are in the side position of the cation [Mim]+,prefer to form stronger hydrogen bonds than the “top” conformers.(2)The optimized structures and interaction energies for 1,3-Dimethyl-imidazolium cation [Mmim]+ complexes,including [Mmim]+[NO3]-? [Mmim]+[ClO4]-? [Mmim]+[BF4]-? [Mmim]+[AlCl4]-?[Mmim]+[Al2Cl7]-,have been studied using the MP2 method with the 6-311++G(d,p)basis set.Comparing to the complexes of 1-methylimidazolium cation [Mim]+,a.the strength of hydrogen bonds of [Mmim]+ is weaker than those of [Mim]+ due to the absence of the protic N-H group;b.the structures of [Mmim]+[AlCl4]-are similar to the [Mmim]+[ Al2Cl7]-,which is similar to the ion-pairs [Mim]+[AlCl4]-and [Mim]+[ Al2Cl7]-,c.the order of intension for hydrogen bonds is: [Mmim]+[NO3]-> [Mmim]+[ClO4]-> [Mmim]+[BF4]-> [Mmim]+[AlCl4]-,which is consistent with the complexes of 1-methylimidazolium cation [Mim]+.2 In this paper,the structures and interaction energies of the complexes 1-methylimidazolium cation [Mim]+?1,3-Dimethyl-imidazolium cation [Mmim]+?ion-pairs [Mim]+[ClO4]-?[Mmim]+[NO3]-with SO2 were systematically studied using B3 LYP methods with the 6-311++G(d,p)basis set.There is one N-H···O-S and one C-H···O-S hydrogen bond between SO2 and [Mim]+,and two C-H···O-S hydrogen bonds between SO2 and [Mim]+.The interaction of the former is stronger by about 14 kJ·mol-1 than the latter.The O,F,Cl atom in the anion can combine with S atom of SO2 to achieve the effect of desulfurization.The results showed that the C-H···O-S hydrogen bonds between acidic hydrogen H atoms of cation and O atom of SO2 and the staticelectrical interaction between SO2 and anions are the mostly contribution to achieve the effect of desulfurization.